I used to use Mango's at XBA, but lately I have been using TA...
Go here and put in these filters:
Platform: All ... or adjust if you don't own a device
Release Status: Listed Only
Title Type: Games Only
Played Status: Games I've Not Played
Then, click on completion time to filter from quickest to longest. Unfortunately, there is no region filter so you'll likely see a ton of NTSC-J games on the first page. However, there are also a ton of ID@Xbox. Depending on how much you've played thus far, you can go 3+ pages before you hit something that takes longer than 8 hours to complete.